In the United States, the Federal Communications Commission (FCC) allows a restricted unlicensed use of ultra-wide bandwidth (UWB) signals for wireless communication systems, “First Report and Order,” Feb. 14, 2002. The UWB signals must be in the frequency range from 3.1 to 10.6 GHz, and have a minimum bandwidth of 500 MHz. The FCC order also limits the power spectral density and peak emissions power of UWB signals, e.g. less than −43.1 dBm/MHz.
One modulation method for UWB uses extremely short time pulses to generate signals with bandwidths greater than 500 MHz, e.g., 1/1,000,000,000 of a second of less, which corresponds to a wavelength of about 300 mm. Systems that use short pulses are commonly referred to as impulse radio (IR) systems.
As shown in FIG. 1A, four different modulation techniques can be used for wireless communication systems, pulse position modulation (PPM) 11, pulse amplitude modulation (PAM) 12, on-off keying (OOK) 13, and bi-phase shift keying (BPSK) 14.
As an advantage, UWB systems can achieve high data rates, and are resistant to multi-path impairments due to the large processing gains. Additionally, the use of IR based UWB technology allows for the implementation of low cost, low duty cycle, low power transceivers that do not require local oscillators for heterodyning. Because UWB radios are primarily digital circuits, they can easily be integrated in a semiconductor chip. In UWB systems, multiple users can simultaneously share the same spectrum with no interference to one another, and the systems are ideal for high-speed home and business networking devices, as well as sensor networks.
In a sensor network, it is desirable to enable the direct communication among multiple inexpensive sensing devices. The IEEE 802.15.4a standard defines a physical-layer for communications with scalable data rates from 1 kbps to 1 Mbps, “IEEE P802.15.4a WPAN Alternate PHY-PAR,” 2003, for low power, low data rate network.
Generally, IR systems are either time-hopped (TH-IR), or transmitted-reference (TR-IR). Both systems use sequences of short duration pulses, p(t). However, the modulation and demodulation for TH-IR and TR-IR differ significantly, making TH-IR and TR-IR incompatible in the same network.
TH-IR system are described by M. Win and R. A. Scholtz, “Ultra-Wide Band Width Time-Hopping Spread-Spectrum Impulse Radio for Wireless Multiple-Access Communications,” in IEEE Trans. On Communications, Vol. 48, No. 4 Apr. 2000, pp. 679-691. In a TH-IR system, each bit or symbol is represented by Nf pulses, where Nf is a positive integer. The time taken to transmit the bit is Ts. This is called the symbol duration. The time Ts is further partitioned into frames Tf, and the frames are partitioned into chips Tc corresponding typically to a pulse duration. If Nc represents the number of chips in a frame and Nf represents the number of frames in a symbol, then Ts, Tf and Tc are related as followsTs=NfTf=NfNcTc.  (1)
FIG. 1B shows the relationship the symbol time Ts 101, the frame time Tf 102, and the chip time tc 103 for pulses 104 for an example prior art TH-IR waveform 110 for a ‘0’ bit, and a waveform 120 for a ‘1’ bit. Typically, the pulses are spaced pseudo-randomly among the available chips in a frame according to a “time-hopping” code to minimize the effect of multi user interference.
As stated above, the modulation can be binary phase shift keying. With BPSK, each bit b is represented as either a positive or negative one bε{−1,1}. The transmitted signal has the form
                                          s            ⁡                          (              t              )                                =                    ⁢                                    ∑                              i                =                1                            ∞                        ⁢                                          ∑                                  j                  =                  1                                                  N                  f                                            ⁢                                                h                                      i                    ,                    j                                                  ⁢                                  b                                      ⌊                                          i                      /                                              N                        f                                                              ⌋                                                  ⁢                                  p                  ⁡                                      (                                          t                      -                                              j                        ⁢                                                                                                  ⁢                                                  T                          f                                                                    -                                                                        c                          j                                                ⁢                                                  T                          c                                                                                      )                                                                                      ,                            (        2        )            where cj represents the jth value of the TH code, in the range {0, 1, . . . , Nc−1}, and b is the ith modulation symbol. Additionally, an optional sequence denoted as hi,j can be applied to each pulse in the transmitted signal so as to shape the spectrum of the transmitted signal and to reduce spectral lines. The sequence, hi,j, is called a polarity scrambling sequence with values of either +1 or −1. Different amplitudes are possible to give further degrees of freedom in the shaping of the spectrum.
FIG. 2 shows a conventional coherent TH-IR receiver 200. The receiver includes an automatic gain control (AGC) unit 210 coupled to an amplifier 220 that is connected to the receive antenna 230. The receiver also includes synchronization 240, timing control 250, channel estimation 260, MMSE equalizer 270, and decoder 280 units. RAKE receiver fingers 290 input to an adder 295. Each RAKE finger includes a pulse sequence generator, correlator and weight combiner. The RAKE fingers reduce multipath interference. Due to the density of the multipaths in UWB signals, the number of required RAKE fingers can be large to obtain reasonable performance. The output of the adder is equalized and decoded. The typical TH-IR receiver has a significant complexity.
TR-IR systems eliminate the need for a RAKE receiver, R. Hoctor and H. Tomlinson, “Delay-Hopped Transmitted-Reference RF Communications,” IEEE Conference on Ultra Wide Band Width Systems and Technologies, 2002, pp. 265-269.” In a TR-IR system, the information is encoded as phase differences of successive pulses in the sequence. Each symbol in a TR-IR system is a sequence of time-hopped ‘doublets’ or pair of two consecutive pulses. Typically, the first pulse in the pair is referred to as a reference pulse and the second pulse is referred to as a data pulse. The two pulses in each pair are separated by a fixed unit of time Td. Multiple pairs can be transmitted for one information bit. The transmitted waveform has the form
                                          s            ⁡                          (              t              )                                =                    ⁢                                    ∑                              i                =                0                            ∞                        ⁢                                          ∑                                  j                  =                                                            iN                      f                                        2                                                                                                              (                                              i                        +                        1                                            )                                        ⁢                                                                  N                        f                                            2                                                        -                  1                                            ⁢                                                h                                      i                    ,                    j                                                  (                                                                  ⁢                                                      p                    ⁡                                          (                                              t                        -                                                  2                          ⁢                          j                          ⁢                                                                                                          ⁢                                                      T                            f                                                                          -                                                                              c                            j                                                    ⁢                                                      T                            c                                                                                              )                                                        +                                                                                                                            ⁢                                          b                                              ⌊                                                  2                          ⁢                                                      j                            /                                                          N                              f                                                                                                      ⌋                                                              ⁢                                          p                      ⁡                                              (                                                  t                          -                                                      2                            ⁢                            j                            ⁢                                                                                                                  ⁢                                                          T                              f                                                                                -                                                                                    c                              j                                                        ⁢                                                          T                              c                                                                                -                                                      T                            d                                                                          )                                                                                            )                                                    ,                            (        3        )            where Tf, Tc, hi,j and Nf are the same as for the TH-IR case.
FIG. 3 shows the relationship the symbol time Ts 301, the frame time Tf 302, and the chip time Tc 303 for pulses 304 for an example TH-IR waveform 310 for a ‘0’ bit, and waveform 320 for a ‘1’ bit.
FIG. 4 shows a conventional TR-IR receiver 400, which is significantly simpler than the TH-IR receiver of FIG. 2. The receiver includes delay 401, multiplier 402, integrator 403, sampler 407 and decision 404 units. The receiver essentially correlates the received signal 405 with a delayed version 406. Obviously, the TR-IR 400 receiver is less complex than the TH-IR receiver 200. However, the reduced complexity is at the cost of requiring twice the number of pulses, and the additional energy required for the reference pulses, nominally 3 dB or more.
It is clear that the decision to use either TH-IR or TR-IR modulation leads to incompatible system structures. Therefore, it is desired to provide a system structure that works with both TH-IR and TR-IR transceivers, to enable cost, complexity and performance trade-offs within a common wireless network.